Missing marble tiles, rusting signs and a tattered, dirty flag make up the 9/11 memorial in East Meadow, Long Island.VictorAlcorn.com (3)

“Never Forget” has been forgotten.

A startling number of 9/11 monuments around the country have been vandalized or fallen into disrepair, including a $2 million Nassau County memorial that bears the names of heroes and residents killed that day.

Visitors to Eisenhower Park in East Meadow last week found the base of a fountain that displays two 30-foot-tall aluminum towers marred by paint peeling in an eerie, ghost-like shape. Black marble tiles surrounding the structure are missing and boarded up. Others are coming loose, and some are streaked with oozing white adhesive.

Graffiti was scrawled across a section facing the wall of 10 plaques honoring all 344 county residents murdered in the Sept. 11, 2001, terror attacks. The empty fountain had condom wrappers.

“A lot of blood, sweat and tears went into this thing,” said Ian Siegel, president of a group that raised $341,000 in donations for the largely taxpayer-funded monument. “Obviously, it’s not the best way to honor the memory of these heroes and loved ones.”

An area of the monument in East Meadow has been damaged.VictorAlcorn.com

Then-President George W. Bush joined weeping 9/11 families in 2004 to break ground for the memorial, which features salvaged steel beams from the fallen Twin Towers. It opened in 2007, two days before the sixth 9/11 anniversary, and was the largest such monument before the reflecting pools at Ground Zero opened in 2011.

Now the site is plagued with chronic problems, according to Brian Nugent, chief deputy commissioner for the county Department of Parks.

“We’re constantly fixing the tiles,” Nugent said. “Usually in the cold months, the adhesive comes off. I don’t know if the original design took the elements into account.”

Nugent sent crews to the site last week after someone posted photos on Twitter showing a mess of dead leaves, litter and broken concrete.

Patricia Mastandrea, a retiree who frequents the memorial on walks through the park, has noticed workers wiping off graffiti. “It just keeps coming back,” she said, blaming “kids who have no respect for anything.”

Plaques list the names, ages, hometowns, and north or south tower where the residents died, or their badges. They include 78 FDNY Bravest, 10 NYPD officers and five Port Authority cops.

“We would never want to desecrate the memories of those souls,” Nugent said. The county plans to consult engineers to help fix the deterioration, perhaps by replacing the tiles.

Nassau County isn’t the only municipality struggling with a poorly constructed memorial. In Green Bay, Wis., a 9/11 monument donated in 2005 by a private group with a salvaged World Trade Center beam has been rotting for years.

The names of the nearly 3,000 people killed in the attacks are etched in granite around the memorial, which features a replica of the Twin Towers.

But most of the names are no longer legible. It turns out the granite, which soon began to crack and erode, was meant for kitchen countertops — not to endure outside in Midwest winters. Water also seeps into the base.

The names on this Wisconsin memorial are illegible due to poor building materials.

“The city accepted an inferior gift,” Mayor Jim Schmitt told The Post. “This should have been thought through.”

“It’s embarrassing and a symbol of disrespect, though unintentional,” said Chad Bronkhorst, head of the city firefighter union, which hopes to raise funds to rebuild the entire monument.

Disrespect and hostility have damaged a string of 9/11 memorials.

In 2011, the statue of a dog, dedicated to canine units that searched through Ground Zero, was smashed off its stand in a Lindenhurst, LI, park.

In 2012, on the 11th anniversary of 9/11, vandals in San Diego toppled 4-foot steel towers inscribed with the names of fallen firefighters, police and other first responders, whacking it off a steel WTC beam donated by the FDNY. No arrests were made.

A homeless man smeared white paint over the name of 9/11 NYPD hero Moira Smith on Coney Island’s Wall of Remembrance in October 2014. A 15-year-old boy wrote on the wall with spray paint in October 2015.

This Wall of Remembrance at a park in Coney Island has been the target of multiple vandals.Demetrius E. Loadholt Sr.

Vandals have struck three times at a 9/11 memorial in Laguna Beach, Calif. The seaside city, population 25,000, installed it in 2012 after a local firefighter obtained a WTC beam from the Port Authority program to distribute pieces for public display.

In the latest incident in February, a baseball-sized dent was smashed into a reflective stainless-steel sphere, which was violently yanked, but not removed, from a welded base that broke in half. The city plans to put up security cameras.

“It feels like somebody doesn’t like what it represents,” said Jorg Dubin, the artist who designed, built and repeatedly repairs the memorial.

Sculptor Jorg Dubin must repeatedly fix dents on this 9/11 monument in California.

Last year, someone tried to steal the sphere. The vandal “worked it back and forth so hard it almost broke off,” Dubin said. In 2013, someone dented it with a pointed piece of metal.

“It’s frustrating for us because there are no leads for us to look into,” said Laguna police spokesman Tim Kleiser.

But one vandal got off scot-free. Homeless artist Salvador Perez, 35, was arrested in Lafayette, La., on the 12th anniversary of 9/11 for defacing a downtown monument by attaching models of exploding airplanes on two twisted, 13-foot steel WTC beams.

A grand jury declined to indict Perez because he didn’t damage the structure and was expressing his right to free speech.